Our general interest is in cellular control mechanisms and in the way in which heredity may modify cellular responses. The synthesis of biologically active proteins normally produced only during embryonic development by certain neoplasms and cells in tissue culture offers an opportunity to investigate the regulation of gene activation. Some of these fetal proteins differ in their chemical and physical properties from the adult forms of the protein and these altered properties may contribute to the malignant behavior of certain tumors which synthesize them. The molecular events which control alkaline phosphatase activity in human cell cultures provide a model for exploring the effects of chemical mediators including hormones on the regulation of an enzyme in a controlled external environment, unaffected by interactions with other cell types. Mechanisms of regulation similar to those observed in this cell system appear to have relevance to regulatory effects described in the course of embryological development and the failure of normal control mechanism seen in neoplasia.